Group A streptococci (GAS) are among the most prevalent of bacterial pathogens, infecting only humans. A hallmark feature of GAS is its molecular and biological diversity among strains. Although GAS can give rise to serious illness, such as autoimmune sequelae and severe invasive disease, most often it causes only a mild infection at superficial sites - the oropharynx (strep throat) or epidermis (impetigo). The throat and skin are the primary tissue reservoirs for GAS, whereby the organism is most successful in reproductive growth and transmission to new hosts. Based on decades of field work, it has become widely recognized that many strains differ in their tissue site preference, giving rise to the concept of distinct throat and skin strains. This recognition strongly suggests that there is an underlying genetic organization that imparts a higher biological order to the diverse strains of this species. A long-term goal is to determine the molecular basis for throat and skin tropisms among GAS. In Preliminary Findings, a diverse set of throat and skin strains were analyzed for linkage disequilibrium between different combinations of loci/alleles, leading to the identification of genes which are likely to be critical for tissue-specific adaptations. The Specific Aims of this proposal are to test the role of the tissue-specific virulence factor candidates in an in vivo mouse model that closely mimics superficial skin infection in humans. Isogenic mutants, and chimeric GAS constructs having a throat gene on a skin strain background, will be constructed and compared for virulence. GAS products to be tested for tissue-specific functions include streptokinase (a plasminogen activator), plasminogen-binding M protein (PAM), the cysteine proteinase SpeB, and transcriptional regulators of GAS virulence gene expression (RofA/Nra and Mga). A better understanding of the molecular basis for throat- and skin-specific infection might aid in the development of vaccines which target essential adaptive traits and thereby, break the chain of transmission.